The present invention relates to metal/fiber composite materials, and in particular, to titanium matrix composites.
In recent years, material requirements for advanced aerospace applications have increased dramatically as performance demands have escalated. As a result, mechanical properties of monolithic metallic materials such as titanium often have been insufficient to meet these demands. Attempts have been made to enhance the performance of titanium by reinforcement with high strength/high stiffness filaments.
Titanium matrix composites have for quite some time exhibited enhanced stiffness properties which approach rule-of-mixtures (ROM) values. However, with few exceptions, both tensile and fatigue strengths are well below ROM levels and are generally very inconsistant.
These titanium composites are fabricated by superplastic forming/diffusion bonding of a sandwich consisting of alternating layers of metal and fibers. At least four high strength/high stiffness filaments or fibers for reinforcing titanium alloys are commercially available: silicon carbide, silicon carbide-coated boron, boron carbide-coated boron and silicon-coated silicon carbide. Under superplastic forming conditions, the titanium matrix material can be made to flow without fracture occurring, thus providing intimate contact between layers of the matrix material and the fiber. The thus-contacting layers of matrix material bond together by a phenomenon known as diffusion bonding. At the same time a reaction occurs at the fiber-matrix interfaces, giving rise to what is called a reaction zone. The compounds formed in the reaction zone may include TiSi, Ti.sub.5 Si, TiC, TiB and TiB.sub.2. The thickness of the reaction zone increases with increasing time and with increasing temperature of bonding. Titanium matrix composites have not reached their full potential, at least in part because of problems associated with instabilities of the fiber-matrix interface. The reaction zone surrounding a filament introduces new sites for crack initiation and propagation within the composite, which operates in addition to existing sites introduced by the original distribution of defects in the filaments. It is well established that mechanical properties are influenced by the reaction zone, that, in general, these properties are degraded in proportion to the thickness of the reaction zone.
It is, therefore, an object of the present invention to provide improved titanium composites.
It is another object of this invention to provide an improved method for fabricating titanium composites.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from a reading of the following description of the invention and the appended claims.